1. Field of the Invention
The present invention relates to an image forming apparatus provided with a large number of surface conduction electron emission devices and to a characteristics adjustment method for an image forming apparatus, a manufacturing method for an image forming apparatus and a characteristics adjustment apparatus that are preferably applied to such an image forming apparatus.
2. Related Background Art
Up to now, there have been known two types of electron-emitting devices, namely, a hot cathode device and a cold cathode device. As the cold cathode device, for example, a field emission device, a metal/insulator/metal electron-emitting device and a surface conduction electron emission device are known.
Among the electron-emitting devices known as the cold cathode device, the surface conduction electron emission device (hereinafter also referred to simply as device) utilizes a phenomenon that electron emission is generated by flowing an electric current to a thin film of SnO2, Au, In2O3/SnO2, carbon or the like of a small area, which is formed on a substrate, in parallel with the surface of the film.
The conventional surface conduction electron emission device will be described with reference to FIG. 17. FIG. 17 illustrates a structure of the conventional surface conduction electron emission device. In the figure, reference numeral 3001 denotes a substrate and 3004 denotes an electroconductive thin film consisting of metal oxide formed by spattering. The electroconductive thin film 3004 is formed in a flat H-shape as illustrated.
An electron-emitting region 3005 is formed by applying an energization operation called energization forming to the electroconductive thin film 3004. An interval L and an interval W in the figure are set to be 0.5 to 1 (mm) and 0.1 (mm), respectively.
Note that, although the electron-emitting region 3005 is shown in the center of the electroconductive thin film 3004 in a rectangular shape for convenience of illustration, this is only schematic and does not represent an actual position or shape of an electron-emitting region faithfully.
As already described, in forming an electron-emitting region of a surface conduction electron emission device, an operation for flowing an electric current to an electroconductive thin film to destroy or deform or deteriorate the thin film locally and form a crack (energization forming operation) is performed.
It is possible to improve an electron-emitting characteristic significantly by further performing an energization activation operation thereafter.
That is, this energization activation operation means an operation for energizing an electron-emitting region, which is formed by the energization forming operation, under appropriate conditions to cause carbon or carbon compound to deposit in its vicinity.
For example, a pulse of a predetermined voltage is periodically applied in a vacuum atmosphere in which organic matter of an appropriate partial pressure exists and a total pressure is 10−2 to 10−3 (Pa), whereby any one of monocrystal graphite, polycrystal graphite and amorphous carbon or mixture of them is deposited in the vicinity of an electron-emitting region to have a thickness of approximately 500 (angstroms) or less.
Note that it is needless to mention that this condition is merely an example and should be appropriately changed according to a material or a shape of a surface conduction electron emission device.
By performing such an operation, an emission current under the same applied voltage can be typically increased to approximately 100 times or more as large as that immediately after energization forming.
Therefore, in manufacturing a multi-electron source that utilizes the above-mentioned large number of surface conduction electron emission devices, it is also desirable to apply the energization activation operation to each device. (Note that it is desirable to reduce the partial pressure of organic matter in the vacuum atmosphere after finishing the energization activation. This is called a stabilization process.)
FIG. 18 is a typical graph of an emission current Ie to device applied voltage Vf characteristic and a device current If to device applied voltage Vf characteristic of a surface conduction electron emission device. Here, in this specification, an emission current means a current that flows between an electron-emitting device and an anode because an electron, which is emitted into a space when the electron-emitting device is driven, is attracted to and collides against the anode if an acceleration voltage is applied to the anode.
Further, the emission current Ie is extremely small compared with the device current If and it is difficult to illustrate them in an identical scale. In addition, these characteristics change when design parameters such as a size and a shape of a device is changed. Thus, two graphs are shown by arbitrary units, respectively.
A surface conduction electron emission device has three characteristics with respect to the emission current Ie as described below.
When a voltage equal to or higher than a certain voltage (which is called threshold voltage Vth) is applied to the device, the emission current Ie increases steeply. On the other hand, the emission current Ie is hardly detected under a voltage lower than the threshold voltage Vth.
That is, the device is a nonlinear device having the clear threshold voltage Vth with respect to the emission current Ie.
Since the emission current Ie changes depending on the voltage Vf applied to the device, a magnitude of the emission current Ie can be controlled by the voltage Vf.
Since a response speed of the current Ie emitted from the device to the voltage Vf applied to the device is high, an amount of charges of electrons emitted from the device can be controlled according to a length of time during which the voltage Vf is applied.
For characteristic adjustment of the surface conduction electron emission device, as described in Japanese Patent Application Laid-Open No. 10-228867 and the like, characteristics of each device can be adjusted by applying a voltage equal to or higher than a certain voltage (which is called threshold voltage Vth) to the device, that is, by applying a characteristic shift voltage (hereinafter also referred to simply as shift voltage) for adjusting characteristics.
Incidentally, a surface conduction electron emission device has an advantage in that a large number of devices can be formed over a large area because it has a simple structure and is easily manufactured.
Thus, image forming apparatuses such as an image display apparatus and an image recording apparatus, an electron beam source and the like, to which a surface conduction electron emission device is applied, have been studied.
The inventors have examined surface conduction electron emission devices of various materials, manufacturing methods and structures. Moreover, the inventors have studied a multi-electron beam source (also referred to simply as electron source), in which a large number of surface conduction electron emission devices are arranged, and an image display apparatus to which this electron source is applied.
For example, the inventors have attempted to manufacture an electron source according to an electric wiring method shown in FIG. 19. FIG. 19 is a view explaining matrix wiring of a conventional multi-electron source.
In FIG. 19, reference numeral 4001 denotes schematically shown surface conduction electron emission devices; 4002 denotes row direction wiring; and 4003 denotes column direction wiring. In the figure, wiring resistances are denoted by 4004 and 4005.
The wiring method as described above is called passive matrix wiring. Note that, although the wiring is shown as a 6×6 matrix for convenience of illustration, a size of the matrix is not limited to this of course.
In the electron source in which devices are arranged in passive matrix, an appropriate electric signal is applied to the row direction wiring 4002 and the column direction wiring 4003 in order to output a desired emission current. In addition, at the same time, a high voltage is applied to an anode electrode (not shown).
For example, in order to drive arbitrary devices in matrix, a selection voltage Vs is applied to terminals of the row direction wiring 4002 of rows to be selected, and at the same time, a non-selection voltage Vns is applied to terminals of the row direction wiring 4002 of rows not to be selected.
In synchronous with this, modulation voltages Ve1 to Ve6 for outputting emission currents are applied to terminals of the column direction wiring 4003. According to this method, voltages of Ve1-Vs to Ve6-Vs are applied to the devices to be selected and voltages of Ve1-Vns to Ve6-Vns are applied to the devices not to be selected.
Here, if Ve1 to Ve6, Vs and Vns are set to appropriate magnitudes such that a voltage equal to or higher than the threshold voltage Vth is applied to the devices to be selected and a voltage equal to or lower than the threshold voltage Vth is applied to the devices not to be selected, an emission current of a desired strength is outputted only from the devices to be selected.
Therefore, the multi-electron source in which surface conduction electron emission devices are arranged in passive matrix has a possibility that it can be applied in various ways. For example, if an electric signal according to image information is appropriately applied, the multi-electron source can be preferably used as an electron source for an image display apparatus.
The multi-electron source manufactured in this way causes slight fluctuation in an emission characteristic of respective electron sources due to variation in a process, or the like.
Such a multi-electron source is preferable for manufacturing a flat image forming apparatus of a large screen. However, since there are a large number of electron sources unlike a CRT or the like, if an image forming apparatus is manufactured using this, there is a problem in that fluctuation of characteristics of respective electron sources appears as fluctuation of luminance.
As described above, as reasons why an electron emission characteristic in a multi-electron source is different for each electron source, various causes are possible such as fluctuation of components of a material used in an electron emitting region, an error of a dimension and shape of each member of the device, nonuniformity of energization conditions in an energization forming operation, and nonuniformity of energization conditions and an atmospheric gas in an energization activation process.
However, a highly advanced manufacturing facility and an extremely strict process management are required if it is attempted to remove all of these causes. If these are satisfied, manufacturing costs increase enormously. Thus, it is not realistic to remove all of these causes.
In Japanese Patent Application Laid-Open No. 10-228867 and the like, a method is disclosed which provides a process of measuring respective characteristics in order to control the fluctuation and a process of applying a characteristic shift voltage for adjusting a characteristic to obtain a value corresponding to a reference value.
However, in the process of measuring characteristics in the invention disclosed in Japanese Patent Application Laid-Open No. 10-228867 and the like, as shown in FIG. 20 (flow chart), there is a process of selecting a device (step 2007), applying a voltage to measure the emission current Ie and luminance (step 2004), saving a result of the measurement in a memory (step 2005) and repeating this measurement operation for all the devices (step 2008). FIG. 20 is a flow chart of a characteristics measurement process in a characteristic adjustment method of the conventional invention.
It is likely that such a process of measuring characteristics of devices for each device takes a long time if the process is used in a high resolution image forming apparatus such as a high definition TV these days, that is, if the number of pixels is large.
Moreover, if luminance is used as a parameter indicating an indicator of nonuniformity, there is an effect that fluctuation of a partial light-emitting characteristic of a phosphor can also be corrected. However, if P22 that is a phosphor generally used in a CRT is used, the red phosphor has {fraction (1/10)} afterglow time of approximately 10 μs for green and blue and 1 ms for red.
If light emission from one device is measured using an optical system one by one, since there is the afterglow time, it is necessary to set a time interval for driving a certain device and the next device to be equivalent to at least the afterglow time.
Therefore, if a high definition display having pixels of approximately 1,280×RGB×768 is constituted, it takes a long time, approximately 1,000 seconds, for measuring all the points.